y also be due to the necessity for greater capacity than
can be secured with the existing battery tanks, usually caused by
underestimating the traffic the office will be required to handle.
Again, it is sometimes necessary to make extensive alterations in an
existing battery, perhaps due to the necessity for changing its
location. To change a battery one cell at a time, keeping the others in
commission meanwhile, has often been done, but it is always expensive
and unsatisfactory and is likely to shorten the life of the battery, due
to improper and irregular forming of the plates during the initial
charge. The advent of the electric automobile industry has brought with
it a convenient means for overcoming this difficulty. Portable storage
cells for automobile use are available in almost every locality and may
often be rented at small cost. A sufficient number of such cells may be
temporarily installed, enough of them being placed in multiple to give
the necessary output. By floating a temporary battery so formed across
the charging mains and running the generators continuously, a temporary
source of current supply may be had at small expense for running the
exchange during the period required for alterations. Usually a time of
low traffic is chosen for making the changes, such as from Saturday
evening to Monday morning. Very large central-office batteries, serving
as many as 6,000 lines, have thus been taken out of service and replaced
without interfering with the traffic and with the use of but a
comparatively few portable cells. One precaution has to be observed in
such work, and that is not to subject the portable cells to too great an
overcharge, due to the great excess of generator over battery capacity.
This is easily avoided by watching the ammeters to see that the input is
not in too great excess of the output, and if necessary, by frequently
stopping the machines to avoid this.

=Power Switchboard.= The clearing-house of the telephone power plant is
the power board. In most cases, it carries switches, meters, and
protective devices.

_Switches._ The switches most essential are those for opening and
closing the motor and the generator circuits of the charging sets and
with these usually are associated the starting rheostats of the motors
and the field rheostats of the generators. The starting rheostats are
adapted to allow resistance to be removed from the motor armature
circuit, allowing the armature to gain sp